I was awarded a 3-year AD last year and am almost done with freshman year. From my experience, I didn't contract as soon as got to school, the only difference between myself and an other cadet with no scholarship was I had a file waiting for me, like cadets that were contracting. That being said strive and talk to your advisor (in the battalion, usually your MS class instructor) about wanting to compete for a 3.5 year scholarship. I was fortunate enough to do so and contracted last Jan. with an other cadet.
There are some forms that you look over, just to make sure all of the information is correct so that when you do contract its all in order.
As for the ceremony, I want to say it varies by battalion. For me, it was before our weekly lab on Thursday while we were in uniform in front of the battalion. Other cadets in battalion contracted before PT in front of the battalion.
Good Luck and congratulations on your scholarship.